1. Field of the Invention
The present invention relates generally to planar optical components used for optical communications, and more particularly to planar optical components employing a waveguide having a core with a relatively high refractive index and/or a waveguide defined by a core and a surrounding cladding in which the difference between the refractive index of the core and the refractive index of the cladding is relatively high.
2. Technical Background
Planar optical components having a waveguide core with a refractive index considerably higher than the surrounding cladding are known. These components comprise a planar substrate onto which an undercladding layer is disposed or a planar substrate which itself acts as an undercladding layer, a patterned core material disposed on the undercladding layer that defines an optical waveguide circuit, and, optionally, an overcladding layer which, together with the undercladding layer, surround the patterned core. An example of a planar optical component having a waveguide defined by a patterned core material having a relatively high refractive index is a photo-optical switch. Due to the relatively high refractive index of the core material in such components, the cross-sectional dimensions (i.e., width and height) of the core are typically much lower than the cross-sectional dimensions of a typical core material having a refractive index, such as about 1.4. In general, the reduced cross-sectional dimensions are necessary to maintain single-mode light propagation through the waveguide since multi-mode propagation associated with larger cross-sectional dimensions results in unacceptable losses of light intensity (i.e., loss of signal and a decrease in the signal-to-noise ratio).
Known planar optical components employing a core material having a relatively high refractive index, and/or a relatively high difference between the refractive index of the core and the refractive index of the surrounding cladding, even when properly constructed to prevent multi-mode light propagation, exhibit relatively high losses of signal strength, as compared with typical waveguides defined by a core having a relatively lower index of refraction, relatively larger cross-sectional dimensions, and a relatively high difference between the refractive index of the core and the refractive index of the cladding. This results since light waves propagated through a waveguide defined by a core material having a relatively high refractive index strike the interface between the core and the cladding more frequently as compared to a typical waveguide defined by a core material having a relatively lower refractive index. Whenever light strikes an interface between the core and the cladding of the waveguide, light may be scattered due to defects at the interface. The higher frequency at which the light strikes the interface between the core and the cladding therefore results in more light being scattered at defects at the interface, and higher losses of signal strength in optical components having a core with a relatively high refractive index and/or a relatively high difference in the refractive index of the core and the refractive index of the cladding.
In addition to the disadvantages associated with higher losses of signal strength in the known planar optical components that include a core having a relatively high refractive index and/or a relatively high difference between the refractive index of the core and the refractive index of the cladding, there are additional losses associated with coupling light into the component from a typical waveguide, such as a standard optical fiber having a cross-sectional dimension (diameter) that is much larger than the cross-sectional dimensions of the high refractive index core material of a planar optical component. Typically, an optical fiber is connected to the input side of a waveguide of a planar optical component by abutting a surface at an end of the optical fiber with a surface at the end of the optical component, and adhering the end of the optical fiber to the optical component. The abutting surfaces are substantially perpendicular to the optical path through the optical fiber and through the waveguide of the planar optical component so that the core of the optical fiber is aligned with the core of the planar optical component as closely as is possible to maximize core-to-core interfacial area, and minimize losses at the interface between the optical fiber and the planar optical component. However, due to the relatively large differences between the cross-sectional dimensions of the core of a typical optical fiber and the cross-sectional dimensions of the core of a typical optical component having a high refractive index core, losses are relatively high at the interface between the optical fiber and the planar optical component. Another contributing factor to the high losses at the interface between the optical fiber and the planar optical component is due to light being reflected at the interface because of the relatively large difference between the refractive index of the core of the optical fiber and the refractive index of the core of the planar optical component.